Process to produce inorganic hollow fibers

ABSTRACT

Process for the production of small tubing, e.g., hollow fibers comprising 
     (a) preparing a solution of an organic fiber-forming polymer, containing, in a uniformly dispersed form, a sinterable inorganic material; 
     (b) extruding the inorganic material-containing polymer solution through a hollow fiber spinneret; 
     (c) forming a polymeric precursor hollow fiber, laden with the inorganic material; 
     (d) treating the polymeric precursor hollow fiber to remove the organic polymer; and 
     (e) sintering the resulting inorganic material in hollow fiber form.

This application is a continuation-in-part of copending application Ser.No. 624,076, filed Oct. 20, 1975, now U.S. Pat. No. 4,104,445 which isincorporated herein by reference.

This invention relates to a process to produce small tubing, e.g.,hollow fibers. Metal tube drawing procedures to make small tubing areexpensive. Such procedures to make extremely small tubing, i.e., withfiber size outer diameters, are particularly expensive and may not betechnically viable. This invention provides a process that readily andeconomically produces metal tubing of extremely small size. The processhas also been found to be useful to produce small tubing of otherinorganic materials.

The value of the process of this invention varies, generally, in inverseproportion with the outer diameter of the small tubing. That is, thesmaller the tubing desired the move valuable the process. For very smallouter diameter tubing, the costs of the process of the present inventiondo not apparently increase per unit length which contrasts with thecosts of tube drawing procedures which generally accelerate whenproducing such small outer diameters.

In the description of the present invention, the following definitionsare used.

The term "hollow fiber" as used in this application means a fiber (ormonofilament) which has a length which is very large as compared to itsdiameter and has an axially disposed continuous channel which is devoidof the material that forms the fiber (more commonly referred to as the"bore"). Such fibers can be provided in virtually any length desired forthe use intended.

The phrase "essentially inorganic materials" denotes a sinterableinorganic material that is substantially free of organic polymericmaterial.

The term "monolithic" means that the material of the fiber has the samecomposition throughout its structure with the fiber maintaining itsphysical configurations due to the presence of sintered particles.

The term "porous" refers to that characteristic of the fiber wall which,although otherwise being continuously relatively dense, has very small,often tortuous, passageways that permit the passage of fluid through thefiber wall other than by diffusion.

SUMMARY OF THE INVENTION

The present invention provides a process to produce essentiallyinorganic, monolithic hollow fibers (i.e., small tubing). Such hollowfibers comprising metal are particularly preferred. The process forproducing such fibers comprises (a) preparing a solution of an organicfiber-forming polymer, containing, in uniformly dispersed form, asinterable inorganic material; (b) extruding the inorganicmaterial-containing polymer solution through a hollow fiber spinneret;(c) forming a polymeric precursor hollow fiber laden with the inorganicmaterial; (d) treating the polymeric precursor hollow fiber to removethe organic polymer; and (e) sintering the resulting inorganic materialin hollow fiber form. The essentially inorganic hollow fiber producedwill be similar to the polymeric precursor hollow fiber but on a reducedscale.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The hollows fibers provided by the present invention will be very usefulto workers in numerous fields. These hollow fibers can be preparedrelatively economically with widely varying physical configurationswhile utilizing many types of inorganic materials. Furthermore, it hasbeen found that large amounts of these fibers can be produced with onlynominal losses due to flaws and imperfections.

The hollow fibers produced by the process of this invention compriseessentially inorganic materials which are sintered in hollow fiber form.The sinterable inorganic materials comprise a very large group ofmaterials. The preferred sinterable inorganic materials are metals.Nickel, iron and their alloys are particularly useful. The sinterableinorganic materials can be ceramics, such as aluminum oxide,beta-alumina, etc. The sinterable inorganic materials can be cermets ormetcers, such as iron metal/aluminum oxide, titanium carbide/nickel,etc.

The hollow fibers produced can have an outer diameter of up to about2,000 microns. However, production of fibers of larger outer diameterssuch as 3,000 or 4,000, up to about 6,000 microns, is also contemplated.Generally, the more economically advantageous hollow fibers have anouter diameter of from about 50 to about 700, most preferably from 100to 550, microns. The fibers often have wall thicknesses of from about 20to about 300 microns. More particularly preferred are fibers having wallthicknesses of from about 50 to about 200 microns. The fibers generallyhave a wall thickness to outer diameter ratio of from about 0.5 to about0.03, particularly preferred of from about 0.5 to about 0.1

An extremely important contribution of the present invention is theability to provide inorganic hollow fibers with varying sizes andconfigurations. The size of the fiber can be influenced by the simpleexpedient of changing spinnerets as is well known in the synthetic fiberfield. By varying the extrusion and fiber-forming conditions the fiberwall thickness can also be varied over wide ranges. Thesecharacteristics provide those skilled in the art with a unique abilityto produce hollow fibers tailored for the application of interest.

These features are provided by the process of this invention which isdescribed more particularly below.

Preparation of Polymer Solution Containing Inorganic Material

A mixture which comprises an inorganic material in uniformly dispersedform in a polymer solution is prepared. The polymer solution comprises afiber-forming organic polymer dissolved in a suitable solvent. Ingeneral the concentration of the organic polymer in the solution issufficient to form, when the solution contains the inorganic material,the precursor polymeric hollow fibers by dry and/or wet spinningtechniques. The polymer concentration can vary over a wide range anddepends on the characteristics desired in the resultant hollow fiber.For instance, if hollow fibers having relatively dense walls are desiredthe concentration can be on the low side. On the other hand, if hollowfibers having less dense walls are desired (all other variablesremaining constant) the concentration must be somewhat higher. Themaximum concentration is, of course, limited to that where the polymersolution containing the inorganic material is not amenable to extrusionthrough a spinneret. Correspondingly, the lower limit is where theresultant polymeric precursor hollow fiber does not have sufficientpolymer to maintain its structure. In general, the polymerconcentrations will be from about 5 to about 35% by weight of thepolymer solution. Particularly preferred polymer concentrations are fromabout 10 to about 30%, more particularly preferred 15% to 30%, by weightof the polymer solution.

The nature of the organic polymer employed in the preparation of thepolymeric precursor hollow fiber according to this invention is notcritical; for example, polyacrylonitrile, polymers of acrylonitrile withone or more other monomers polymerizable therewith such as vinylacetate, methyl methacrylate, polyurethanes and polyvinyl chloride maybe used. Both addition and condensation polymers which can be cast,extruded or otherwise fabricated to provide hollow fibers by dry or wetspinning techniques are included. Typical polymers suitable for use inthe process of the present invention can be substituted or unsubstitutedpolymers and may be selected from polysulfones; poly(styrenes),including styrene-containing copolymers such as acrylonitrile-styrenecopolymers, styrene-butadiene copolymers and styrenevinylbenzylhalidecopolymers; polycarbonates; cellulosic polymers, such as celluloseacetate-butyrate, cellulose propionate, ethyl cellulose, methylcellulose, nitrocellulose, etc.; polyamides and polyimides, includingaryl polyamides and aryl polyimides; polyethers; poly (arylene oxides)such as poly(phenylene oxide) and poly(xylylene oxide);poly(esteramidediisocyanate); polyurethanes; polyesters (includingpolyarylates), such as poly(ethylene terephthalate), poly(alkylmethacrylates), poly(alkyl acrylates), poly(phenylene terephthalate),etc.; polysulfides; polymers from monomers having alphaolefinicunsaturation other than mentioned above such as poly(ethylene),poly(propylene), poly(butene-1), poly(4-methyl pentene-1), polyvinyls,e.g., poly(vinyl chloride), poly(vinyl fluoride), poly(vinylidenechloride), poly(vinylidene fluoride), poly(vinyl alcohol), poly (vinylesters) such as poly(vinyl acetate) and poly (vinyl propionate),poly(vinyl pyridines), poly(vinyl pyrrolidones), poly(vinyl ethers),poly(vinyl ketones), poly(vinyl aldehydes) such as poly(vinyl formal)and poly(vinyl butyral), poly(vinyl amines), poly(vinyl phosphates), andpoly(vinyl sulfates); polyallyls; poly(benzobenzimidazole),polyhydrazides; polyoxadiazoles; polytriazoles; poly(benzimidazole);polycarbodiimides; polyphosphazines, etc., and interpolymers, includingblock interpolymers containing repeating units from the above such asterpolymers of acrylonitrile-vinyl bromide-sodium salt ofparasulfophenylmethallyl ethers; and grafts and blends containing any ofthe foregoing. Typical substituents providing substituted polymersinclude halogens such as fluorine, chlorine and bromine; hydroxylgroups; lower alkyl groups; lower alkoxy groups; monocyclic aryl; loweracyl groups and the like.

Furthermore, since the organic polymer is to be treated to remove it insubsequent steps of the process, it should be amenable to thistreatment. For instance, a more preferred polymer would be one thatreadily decomposes and/or reacts, but not at an excessively rapid rate,to effect its removal. Still further, such polymers should not formreaction products that will adversely interact with the inorganicmaterials or interfere with the subsequent steps in the process.

Obviously the cheapest and most readily available polymers arepreferred. Polymers and polymers of acrylonitrile with one or moremonomers polymerizable therewith or particularly amenable to the processof this invention.

The solvents to be used in the preparation of the polymer solution canbe any number of those well known to one skilled in the art. Forinstance, such solvents as dimethylacetamide, dimethylformamide,dimethyl sulfoxide, etc., are particularly useful with polymers ofacrylonitrile with one or more monomers polymerizable therewith.Obviously the solvent selected should be a good solvent for the organicpolymer and should be amenable to the dry or wet spinning techniquescontemplated in the subsequent steps of the process.

The polymer solution containing an inorganic material can be prepared bydispersing the inorganic material in the solvent followed by theaddition and dissolution of the polymer in the solvent. Any othersuitable means of preparing the polymer solution containing an inorganicmaterial is acceptable, for instance, by concurrently mixing polymer,inorganic material and solvent or by mixing the polymer and the solventfollowed by addition and dispersion of the inorganic material, etc. Itis preferred to disperse the inorganic material in the solvent prior topolymer addition.

Ambient or somewhat higher temperatures are usually quite adequate forthe preparation of the polymer solution containing an inorganicmaterial. Dependent on polymer, solvent and/or inorganic materialutilized higher or lower temperatures may aid the preparation but arenot considered critical.

The amount of the inorganic material is inversely related to the samegeneral considerations discussed above concerning the polymerconcentration in the polymer solution. The maximum amount is limited tothat where the precursor fiber structure can not be maintained becausesufficient polymer is not present. The minimum amount is where theinorganic material particles are so widely dispersed that they do notsufficiently fuse or bond during sintering. Normal ratios, by weight, ofinorganic material to polymer will range from about 3.5 to about 15.Preferred ratios of inorganic material to polymer are from about 4 toabout 12, more preferably from 4.5 to 10.

The inorganic material must be uniformly dispersed as, e.g., smallparticles, throughout the polymer solution. Sufficient mixing must becarried out to achieve such a uniform dispersion. Although some amountof inorganic material may be dissolved, and this may be helpful inachieving a uniform dispersion, this is not critical to achieving theobjectives of the present invention.

The inorganic material incorporated into the polymer solution is asinterable inorganic material (this phrase includes materials from whicha sinterable material can be prepared). Such materials constitute anextraordinarily large group of materials that either are suitable assuch or that can be converted to the desired sinterable inorganicmaterial. For instance, if the desired fiber is to comprise a metal,such as nickel or its alloy, either the metal, its oxide or othercompounds that can be ultimately converted to the metal can be used.

Although the process of the present invention is particularly useful inproducing hollow fibers or metals, such as by the reduction of metaloxides to metal and sintering of the metal, it may be utilized toproduce hollow fibers of any inorganic materials that are sinterable orthat can be converted to a sinterable material. Such inorganic materialsare discussed above. For purposes of illustration, the followingdetailed description will be limited to metal compounds which arereducible to metals and which are sinterable.

Since the reduction temperatures must, of course, be below the meltingand vaporization point of the compounds being reduced and of theelemental metal formed, the metal compounds which vaporize or sublimeexcessively at temperatures below that at which they will react withhydrogen or carbon, the metal component of which has such a lowtemperature of vaporization of sublimation (e.g., K, Na, Li, etc.), maynot be satisfactorily used in accordance with the present processwithout special consideration. Although the use of hydrogen to providethe environment for reducing the metal compound particles to elementalmetal is a preferred embodiment of the present invention, other reducingmaterials may be employed. For example, the metal compounds andparticularly nickel and iron oxides can be reduced by partially orwholly substituting carbon monoxide for the hydrogen reducingenvironment. Obviously the constituents of the polymer and traces ofsolvent will also contribute to such a reducing environment.

Additionally the metal compound itself is limited to those materialswherein the reaction products, other than the elemental metal, willleave the reaction zone prior to or during sintering of the hollowfiber.

The most significant metal compounds are, of course, the oxides sincethese compounds are the most plentiful; and, in fact, are the state inwhich metals are most commonly found as by-products of manufacturing andin natural ore concentrates. Other compounds which may be utilizedinclude metal halides, hydroxides, carbonates, oxalates, acetates, etc.

Particle size is an important factor for producing the desired hollowfibers regardless of the inorganic material utilized. Small particlesutilized for dispersion in the polymer solution usually range in sizefrom less than 15 microns, preferably 10 microns, most preferably 5microns or less. Generally such particles will range in sizedistribution from one end of the scale to the other. Obviously thesmaller particle sizes, i.e., less than 10 microns, are preferred inorder to obtain a uniform dispersion. To obtain metal fibers of desiredcharacteristics it may be necessary to use very small particles, i.e., 5microns or less. This may require particle size comminution and/orclassification to achieve desired sizes.

A generally smaller diameter particle would be expected to intensify"outgassing" cracking and surface problems observed with compactionprocedures since the smaller particles are closer together leaving lessroom for the evolved reaction gases to escape. However, it has beenfound that where the smaller diameter particles are utilized a moreflaw-free hollow fiber can be produced.

A still further difficulty in using very fine metal particles relates tothe tendency of many metals to oxidize when exposed to air in smallparticle form. For example, fine iron particles (40 microns or less)tend to react exothermically when exposed to air to form iron oxideparticles. Thus, it is difficult to handle such materials while theoxide particles can be freely shipped and easily handled withoutproviding air tight protective envelopes or making special provisions toavoid spontaneous reactions. The process of this invention isparticularly amenable to use of oxides since oxide particles are oftenby-products of metal treating, and, consequently, are readily availableat low prices. For example, iron oxide particles obtained as aby-product from hydrochloric acid pickling is readily available. Othersources of iron oxide particles include dust from basic oxygenconverters, rust, mill scale, and high-grade iron ore. Nickel oxide isavailable at nominal prices.

Metal compound particles of any general shape (i.e., spherical, oblong,needles, or rods, etc.) may be employed in accordance with the presentinvention. Metal oxide particles obtained by the process of spray dryinga dissolved metal compound can provide superior hollow fibers.

Accurate particle size determinations of fine-grained particles aredifficult to obtain, particularly where the size includes particles lessthan 10 microns in diameter (or smallest dimension). Such determinationsare most difficult where the particles are of non-uniform shape. Forexample, many of the particles are likely to be of a relativelyelongated configuration so that it is difficult to determine thesmallest dimension of the particle. Elongated particles will not passthrough a screen having a mesh that is designed to accommodate arelatively symmetrically shaped particle of equivalent mass. As a resultparticle size and particle size distribution measurements vary to aconsiderable degree for a given material between the known methods andprocedures for making such determinations.

Relatively accurate fine-grained particle size determinations may bemade through the use of Coulter counter procedure. In this procedure theparticles are suspended in an electrically conductive liquid and arecaused to flow through a small orifice. A current is caused to flowthrough the orifice by means of two immersed electrodes, one on eachside of the orifice. As the particles flow through the orifice, thechange of electrical resistance between the electrodes is measured todetermine particle size. Thus, the measure primarily is interpreted onparticle mass and is not affected by shape.

A particularly desirable feature of the process of the present inventionwhen using metal compounds relates to the "active" state of the metalfiber reduction of the metal compound particles and prior to sintering.Metal particles tend to acquire a thin oxide coating or film and in factnearly all metal powders of fine particle size must acquire or beprovided with such a film to prevent rapid oxidation or defeat thepyrophoric nature of such materials. Such a film renders the particles"passive" so that they may be handled in ordinary atmosphere. However,such a film is difficult to reduce and retards sintering. When metalcompound particles are reduced in accordance with the process of thepresent invention and are sintered subsequent to reduction without beingexposed to an oxidizing environment hollow fibers having excellentproperties may be obtained due to the "active" nature of the reducedparticles. This feature further enhances the value of this invention.

Metal alloys can be provided as the inorganic material of the fiber ofthis invention by the simple expedient of mixing particles of metalcompounds, e.g., metal oxides, and dispersing this mixture in thepolymer solution. Such alloys can provide useful strength and othercharacteristics. Exemplary of such alloys are those formed using nickeland iron oxides.

Another acceptable procedure for making metal hollow fibers by thepractice of the process of the present invention is to incorporate metalparticles with the particulate metal compounds. Preferably the metalparticles will be blended with the metal compounds prior to dispersionin the polymer solution. Reducing and sintering may be accomplished atthe usual temperatures and in the presence of the usual atmospheres (inaccordance with the process of the present invention). The sinteringtemperature may be high enough to effect diffusion of the elementalmetal into the reduced base metal to effect alloying. Consequently, itmay be necessary or desirable to employ a somewhat higher sinteringtemperature where the elemental metal has a low diffusion rate. If thesintering temperature of the elemental metal (or temperature at whichdiffusion of the elemental metal into the base metal will occur) ishigher than the melting point of the base metal then alloying may not beaccomplished. However, in the latter eventuality the elemental metal orits oxide may dispersion strengthen the base metal.

An additional use of metal particles is to reduce shrinkage of thesintered product. In any sintering process, the metal article shrinks inits outer dimensions due to the elimination of the void spaces betweenthe particles when the particles fuse to form a solid mass. When theinorganic material comprises metal compounds such as metal oxides thatare first reduced and then sintered in accordance with the method of thepresent invention such shrinkage is accentuated due to the fact that thereduced particles are smaller than the metal compound particles and thusprovide greater void spaces between particles. Such shrinkage can bereduced or minimized by adding elemental metal particles to the metalcompound particles for incorporation in the polymer solutions. Forexample, it may be desirable to add up to 50 percent, by weight, nickelpowder to nickel oxide powder to reduce shrinkage of the resultanthollow fiber. The particle size of the elemental metal particles willpreferably be very small since such dispersed particles will diffuseinto a matrix metal quickly and evenly.

Further, by including with the metal compound a proportion of dispersed,non-reducible (or diffusible) materials of controlled particle size, itis possible to effect a dispersion strengthened sintered product. Theparticles may consist of elemental metals that sinter at a highertemperature than the sintered product.

As mentioned above, the sinterable inorganic material can be a materialthat comprises the fiber material without chemical modification or amaterial that is converted to a desired form by chemical modification.As extensively discussed above, metal compounds particularly metaloxides, are illustrative of the latter materials. If metal fibers aredesired these oxides require reduction to the elemental metal prior toor during sintering. Other materials that are amenable to the process ofthe present invention are those that may require oxidation or bothoxidation and reduction to form the material comprising the resultanthollow fiber. Although these procedures will not be discussed in thedetail provided for metal compounds, these materials, such as aluminum,are also useful with the process of this invention. Other inorganicmaterials which can be provided by simultaneous oxidation and reductionare also useful in the process of this invention. Illustrative of thesematerials is the simultaneous oxidation and reduction of aluminum ortitanium and iron oxide or nickel oxide. The following materialsillustrative of those materials which can comprise the final fiberswithout chemical modification (i.e., without reduction and/or oxidation)are metals, ceramics such as alumina, beta-alumina, glass, mullite,silica, etc.

The polymer solution containing an inorganic material can also containother additives to assist in this and subsequent steps in the process,particularly for instance, in the extrusion and fiber-forming steps.Surfactants such as sorbitan monopalmitate, etc., are useful to wet theinorganic material by the solvent of the polymer solution. Plasticizerssuch as N,N-dimethyl lauramide, etc., are useful to provide polymericfiber flexibility.

Extrusion of Polymer Solution Containing Inorganic Material

In making hollow fibers by the process of the present invention, a widevariety of extrusion conditions may be employed. As previouslydiscussed, the weight percent polymer in the solution may vary widelybut is sufficient to provide a hollow fiber under the extrusion andfiber-forming conditions. If the inorganic material, polymer and/orsolvent contain contaminants, such as water, particulates, etc., theamount of contaminants should be sufficiently low to permit extrusionand/or not interfere with or adversely affect subsequent steps in theprocess or the resultant fiber. If necessary, contaminants can beremoved from the polymer solution by filtration procedures. Obviouslyfiltration must be appropriate to remove contaminant particles whilepassing the particles of inorganic material. Such filtration may alsoremove particles of inorganic material which are above the desiredparticle size. The presence of excessive amounts of gas in the polymersolution containing inorganic material may result in the formation oflarge voids and undesirable formation of porosity in the precursorpolymeric hollow fiber. Accordingly, degassing procedures are alsoappropriate. Such degassing and/or filtration procedures can be carriedout immediately after or during preparation of the polymer solutioncontaining an inorganic material or can be carried out immediately priorto or during the extrusion step.

The size of the hollow fiber spinnerets will vary with the desiredinside and outside diameters of the resultant polymeric precursor hollowfiber. The spinnerets may also vary in shape, i.e., hexagonal, oblong,star, etc. The spinnerets are generally circular in shape and may haveouter diameters of, for instance, about 75 to about 6000 microns withcenter pin outer diameters of about 50 to about 5900 microns with aninjection capillary within the center pin. The diameter of injectioncapillary may vary within the limits established by the pin. The polymersolution containing the inorganic material is frequently maintainedunder a substantially inert atmosphere to prevent contamination and/orcoagulation of the polymer prior to extrusion and to avoid undue firerisks with volatile and flammable solvents. A convenient atmosphere isdry nitrogen.

The temperature preparatory for extrusion of the polymer solutioncontaining inorganic material can vary over a wide temperature range. Ingeneral the temperature is sufficient to prevent undesirable coagulationor precipitation prior to extrusion. The temperature generally can rangefrom about 15° C. to about 100° C. preferably from about 20° C. to about75° C.

The pressure to accomplish the extrusion is normally those within theranges understood by those skilled in the fiber spinning arts. Thepressure depends on, for instance, the desired extrusion rates, theorifice size and the viscosity of the polymer solution containing theinorganic material. Of particular note is the fact that relatively lowpressures can be utilized with the process of the present invention.This contrasts with compaction procedures which often require hundredsof atmospheres of pressure to provide compacted and sintered articles.The pressure useful with the present invention normally range from about1 atmosphere up to about 5 atmospheres or higher.

Obviously the fibers can be extruded through a plurality of spinnerets.This will enable the concurrent formation of multiple fibers while, forinstance, using the same coagulating bath.

Formation of the Polymeric Precursor Hollow Fiber

In general, fiber-forming spinning techniques are known to those skilledin the synthetic fiber-forming industries. These skills can beadvantageously applied to the fiber-forming step of the process of thisinvention. The fiber-forming step may be conducted using wet or dryspinning techniques, i.e., the spinneret may be in or removed from thecoagulating bath. The wet technique is often preferred and may be usedfor the sake of convenience. That is, the fiber coagulation can beeffected by bringing the fiber which is being formed by extrusion intocontact with a coagulating bath. It suffices to pass the fiber which isbeing formed into the coagulating bath. A fluid which coagulates thepolymer of polymer solution is usually injected into the bore of thefiber being formed. The fluid may comprise, e.g., air, isopropanol,water, or the like.

Any essentially non-solvent for the polymer can be employed as thecoagulating agent in the coagulating bath. The coagulating agent may bemiscible with the solvent. The nature of the coagulating agent selecteddepends on the solvents used for the organic polymer and the choicedepends on criteria known in the field of fiber spinning. It isimportant to use mild coagulating agents for both the bore injectionfluid and in the coagulating bath to obtain uniform density fiber walls.By a "mild coagulating agent" is meant a medium in which the organicpolymer will precipitate slowly so that coagulation does not occurrapidly. Conveniently, water is employed as a coagulating agent at lowconcentrations in the coagulating bath. Other coagulating agents are:ethylene glycol, polyethylene glycol, propylene glycol, methanol,ethanol and propanol, etc. Ethylene glycol is a particularly preferredcoagulating agent. The residence time for the extruded fiber in thecoagulating bath is at least sufficient to ensure reasonablesolidification of the fiber. The fiber wall is formed due to interactionwith the coagulating agents and/or cooling. (Cooling may also beachieved by bringing the extruded polymer solution containing inorganicmaterial into contact with a gas at a temperature below the gellingtemperature of the polymer solution. Where gelling is accomplished inthis manner, the cooling gas can be subjected to a relatively rapidtranslatory movement which can be oriented in a direction parallel tothat of the hollow fiber. This gas may additionally be charged withwater vapor or the vapor of some other non-solvent). Where gelling isalso accomplished in the coagulating bath the bath may, in addition toits gelling effect, also impart a coagulating effect.

The temperature of the coagulating bath may also vary widely, e.g., from-15° to 95° C. or more, and is most often about 1° to 35° C., say, about2° to 25° C. The temperature of the fluid injected into the bore isgenerally within the same ranges.

After coagulating the fiber it may be washed to remove solvent by, forinstance, washing with the coagulating bath solution or with othernon-solvents that are miscible with the solvent of the polymer solution.Washing may cause further coagulation. The precursor hollow fiber mayalso be stored in a water or other liquid bath.

The extrusion and fiber-forming conditions are preferably such that thefiber is not unduly stretched. Although not necessary, stretching can beused say, about 1 to about 5 fold. Frequently, extrusion andfiber-forming speeds are within the range of about 5 to 100 meters perminute although higher speeds can be employed providing the fiber is notunduly stretched and sufficient residence time is provided in thecoagulating bath. Stretching generally strengthens the polymericprecursor hollow fiber. Stretching also allows increased linearproductivity and smaller fiber diameters with a given spinneret.

An annealing procedure may also be carried out to toughen the polymericprecursor hollow fiber. Both the stretching and annealing procedures canbe conducted by, for instance, passing the fiber through boiling water.

The precursor hollow fibers of polymer laden with an inorganic materialcan be subjected to the subsequent steps in the process or can be takenup and stored in precursor form on, for instance, bobbins. The precursorfibers are flexible and have reasonable degree of strength and cantherefore be handled without undue concern for damage.

After obtaining the precursor fiber by the process of the inventiondrying may be carried out in a known manner. The fibers are generally,but not necessarily, dried prior to treatment to remove the organicpolymer. The drying may be conducted at about 0° to 90° C., convenientlyabout room temperature, e.g., about 15° to 35° C., and at about 5 to 95,conveniently about 40 to 60, percent relative humidity.

The precursor hollow fiber comprises the polymer in minor amount actingas the carrier for the inorganic material which is uniformly dispersedthroughout the polymer. Generally, the polymer is present in theprecursor hollow fiber in concentrations substantially less than 50% andoften as low as 25%, 15%, or 5% by weight. The major component in theprecursor fiber being, of course, the inorganic material. Othermaterials may be present in the precursor fiber but generally only insmall amounts.

Treatment to Remove Organic Polymer

After formation of the polymeric precursor hollow fibers laden withinorganic material the fiber can be preferably dried or stored and driedas discussed above, or transferred directly to a treatment to remove theorganic polymer from the fiber. This can be accomplished by heating todecompose and/or react the organic polymer. This may be accomplished inan inert or reducing atmosphere to aid in reduction of the inorganicmaterial, although this is not always necessary.

As mentioned above, the reaction products formed from the organicpolymer may serve to enhance the other steps of the process. Forinstance, the hydrogen and carbon present in the polymer serve as anexcellent source of a reducing environment. This environment helps toreduce metal compounds, e.g., oxides, to the elemental metal.

The fiber containing inorganic material may, optionally, be subjected toreduction and/or oxidation. (It is, of course, recognized that neitherreduction or oxidation may be necessary if the inorganic materialdispersed into the polymer solution is in the chemical form desired forsintering.) Preferably an appropriate atmosphere will be provided justprior to the fiber being subjected to the reduction and/or oxidationtemperature. For instance, with reduction, this may be accomplished bycontinuously passing the polymeric precursor hollow fiber laden with areducible inorganic material through a commercially available oven. Anatmosphere comprising, for instance, hydrogen may be caused to flowcountercurrently and in contact therewith. As the fiber first contactsthe heat of the oven, the remaining volatile components will outgas. Asthe temperature approaches reducing temperatures, the reducibleinorganic material, for instance, metal compounds are converted toelemental metal and the reaction products outgas.

For the purposes of the present invention and this specification, itwill be understood that the temperature range at which polymer removaland reduction and/or oxidation will occur and the sintering temperaturesmay overlap to some extent. In other words, some sintering may occur atthe temperatures at which polymer removal and reduction and/or oxidationis carried out, although it is preferable that the temperature be suchthat reduction takes place immediately preceding sintering. Thepreferred temperatures at which reducible inorganic materials, i.e.,metal compounds will reduce are well-known to those skilled in the artor their determination is well within the skill of those of ordinarycompetency.

The preferred reducing environment may be provided by any atmospherewhich provides a source of hydrogen. For example, such an atmosphere maycomprise pure hydrogen, cracked hydrocarbons, dissociated ammonia,combinations of each, combinations of one or more of such gases andother gases or vapors which will not materially interfere with thereduction reaction. The reaction products from the decomposition and/oroxidizing of the polymer are valuable aids in providing the reducingatmosphere.

Solid reducing materials, carbon for example, may be employed incombination with the hydrogen yielding gas only where the reactants(e.g., CO and CO₂) appropriately "outgas" and will not leave residualelements in the sintered fiber that will interfere with the desiredfiber properties. For example, carbon may be a desired addition to theoxide powder as set forth above where the ultimate product is a steelcomposition and the residual carbon is a necessary element for thefinished fiber.

Oxidation of the inorganic material can be conducted at the appropriatetemperatures under suitable pressures and atmospheres. Air is thepreferred atmosphere. The oxidation temperatures are generallywell-known or readily ascertainable. Simultaneous oxidation andreduction can occur, say, for instance, in the formation of cermets. Theresulting fiber comprising a sinterable inorganic material may then beconducted directly into a sintering zone.

Sintering to Form to Inorganic Fiber

The term "sintering" is meant to include an agglomeration by fusion andbonding of the sinterable inorganic material to at least that point atwhich the particulate material forms a monolithic structure. Sinteringshould provide a fiber having substantial strength as compared to afiber which has undergone the previous steps and has not been sintered.The sintering must be conducted under conditions that assure that thevalence state desired is achieved or maintained under sufficienttemperatures and times to allow the fusion and bonding to occur.

In the production of the hollow fibers of this invention there arelittle or no limitations on the heating rate for sintering. Forinstance, the sintering of a nickel-iron alloy fiber can be at fromabout 950° C. to about 1200° C. for from 15 to 5 minutes, respectively.A nickel-iron alloy fiber produced under these conditions is excellent.In general, similar to the reduction and oxidation temperatures, thepreferred sintering temperatures of the inorganic materials arewell-known or readily ascertainable.

During the organic polymer removal, optional reduction and/or oxidationof the inorganic material and sintering steps, suitable conditions mustbe maintained to avoid damage or destruction to the fiber wall structureand integrity. A shrinkage ratio (final fiber to precursor fiber) offrom about 0.2 to about 0.9 can be expected, usually 0.3 to 0.6. Thatis, the precursor hollow fiber is often transformed to the final hollowfiber with substantial size reduction. This is expected during theseprocess steps. For instance, the fiber is substantially reduced inlength and the fiber outer diameter and wall, although remaining inrelative relationships, are also reduced in size. During these stepsmeans must be provided to handle the fiber as it shrinks. Particularlycritical is the point immediately prior to sintering where the fiber isfairly fragile. At this point, particular care must be taken to providemeans to afford such shrinkage without damage to the fiber. Forinstance, if the fiber is allowed to adhere to a conveying surface atthis point it may break as it shrinks. One method of handling the fiberat this point is to feed a precursor fiber, which may be pretreated,e.g., with water, to provide better handling characteristics, into thefurnace by means of a conveyor belt which is fabricated of materialwhich does not adhere to the fiber under the operating conditions of thefurnace. This conveyor belt can be transporting the fiber at the speedof the final fiber as it exits the furnace. The precursor fiber feedspeed is faster than the final fiber speed. The precursor feed speed canbe adjusted to account for the shrinkage that occurs.

A particularly important feature of the process of this invention is theability to produce fibers having relative strong and dense walls. Thisfeature is surprising since the polymer of the polymeric precursor fiberis the continuous phase which is removed as discussed above. It has beenfound that, although the polymer is removed from the fiber wall of aprecursor fiber, the final fiber, after sintering, is usually quitestrong and dense. Although it might be expected that shrinkage andreduction of interstices between particles of inorganic materials mightoccur when the inorganic material undergoes reduction, oxidation and/orsintering, the formation of a fiber wall that is strong and dense, i.e.,inhibits passage of fluids, is both desirable and unexpected. Thisphenomena appears to occur throughout the fiber wall where ever polymeris removed. It has been observed particularly when using metalcompounds, e.g., oxides, to convert to elemental metal.

The process of this invention can also produce hollow fibers having aporous wall. This can be achieved by, for instance, treating the fiberwall with a fluid that has some interaction with the material of thewall to produce a porous wall. For instance, a polymeric precursor fibercontaining nickel oxide can result in a uniformly porous wall surface byintroducing ammonia gas in the atmosphere in the furnace.

An alternate means to obtain a porous fiber wall is to introduce arelatively small amount of fine particulate material which does notparticipate in the sintering or participates in the sintering to alesser degree. Incorporation of such fine particulate materials in thepolymer solution containing an inorganic material during its preparationcan result in a porous fiber wall in the final inorganic fiber.

The hollow fiber resulting from the process is strong compared toprecursor fiber and fibers from the intervening steps. The final fibersmay be flexible enough to be stored on bobbins.

It is claimed:
 1. A process comprising(a) preparing a solution of anorganic fiber-forming polymer, containing, in a uniformly dispersedform, a sinterable inorganic material; (b) extruding the inorganicmaterial-containing polymer solution through a hollow fiber spinneret;(c) forming a polymeric precursor hollow fiber, laden with the inorganicmaterial; (d) treating the polymeric precursor hollow fiber to removethe organic polymer; and (e) sintering the inorganic material; providedthat steps (d) and (e) are conducted under conditions that maintain ahollow fiber form.
 2. The process according to claim 1 wherein theinorganic material dispersed in the polymer solution comprises a metalcompound which is reduced prior to or during sintering.
 3. The processaccording to claim 2 wherein the inorganic material dispersed in thepolymer solution comprises a metal oxide.
 4. The process according toclaim 3 wherein the metal oxide dispersed in the polymer solutioncomprises nickel oxide or nickel oxide and an oxide of a metal thatforms a nickel alloy.
 5. The process according to claim 4 wherein themetal oxide that forms a nickel alloy is iron oxide.
 6. A processaccording to claims 1, 2, 3, 4 or 5 wherein the inorganicmaterial-containing polymer solution is extruded directly into acoagulating bath.
 7. A process according to claim 6 wherein acoagulating agent is injected into the bore of the fiber as it isextruded.
 8. A process according to claim 6 wherein the coagulatingagent comprises water.
 9. A process according to claims 1, 2, 3, 4 or 5wherein the inorganic material-containing polymer solution passesthrough a gas before contacting the coagulating bath.
 10. A processaccording to claim 9 wherein a coagulating agent is injected into thebore of the fiber as it is extruded.
 11. A process according to claim 9wherein the coagulating agent comprises water.
 12. A process accordingto claim 6 wherein the coagulating agent comprises ethylene glycol. 13.A process according to claim 9 wherein the coagulating agent comprisesethylene glycol.